In recent years, a nondestructive sensing technology using a terahertz wave has been developed. As an application field of an electromagnetic wave having this frequency band, there is an imaging technology with a safe fluoroscopy device instead of an X-ray equipment. In addition, there have been developed a spectral technology for investigating physical properties such as a molecular binding state by determining absorption spectrum and complex permittivity inside a substance, a measurement technology for investigating physical properties such as carrier density, mobility, and conductivity, and an analysis technology of biomolecules. As a method of generating a terahertz wave, a method of using a nonlinear optical crystal is widely used. Typical nonlinear optical crystals include LiNbOx (hereinafter, also referred to as LN), LiTaOx, NbTaOx, KTP, DAST, ZnTe, GaSe, GaP, CdTe, and the like. A secondary nonlinear phenomenon is used for generating a terahertz wave. As the method, there are known a difference-frequency generation (DFG) using incidence of two laser beams having a frequency difference. In addition, there are known a method of generating a single color terahertz wave by an optical parametric process and a method of generating a terahertz pulse by optical rectification with irradiation of a femtosecond pulse laser beam.
As a process of generating a terahertz wave from a nonlinear optical crystal in this way, an electrooptic Cerenkov radiation has been noted recently. This is a phenomenon in which, as illustrated in FIG. 9, a terahertz wave 101 is radiated in a conical manner like a shock wave in a case where a propagation group velocity of a laser beam 100 as an excitation source is faster than a propagation phase velocity of the generated terahertz wave. A radiation angle θc of the terahertz wave is determined by the following equation according to a ratio of refractive index in the medium (nonlinear optical crystal) between light and the terahertz wave.cos θc=vTHz/vg=ng/nTHz where a group velocity and a group refractive index of the laser beam are denoted by vg and ng, respectively. A phase velocity and a refractive index of the terahertz wave are denoted by vTHz and nTHz, respectively. Up to now, there has been reported that a high intensity terahertz pulse is generated by optical rectification using the Cerenkov radiation phenomenon by causing a femtosecond laser beam with inclined wavefront to enter LN (see Non Patent Literature 1). In addition, there has been reported that a single color terahertz wave is generated by a DFG method using a slab waveguide having a thickness sufficiently smaller than the wavelength of the generated terahertz wave in order to eliminate the necessity of the wavefront inclination (see Patent Literature 1 and Non Patent Literature 2).
The examples of Patent Literature 1, Non Patent Literature 1, and Non Patent Literature 2 are related to a proposal of, since the terahertz wave is generated by progressive wave excitation in those examples, improving extraction efficiency by enhancing terahertz waves generated by different wave sources by each other with phase matching in the radiation direction. Features of this radiation method include the fact that a high intensity terahertz wave can be generated with relatively high efficiency as the ones using a nonlinear optical crystal, and the fact that a terahertz wave band can be widened when absorption in the terahertz region due to phonon resonance unique to the crystal is selected on the high frequency side. In those technologies, compared with terahertz generation by using a photoconduction element, generation band can be widened and the pulse width can be decreased in the case of terahertz pulse generation using the optical rectification. Therefore, it is expected that device performance can be enhanced in the case of application to a terahertz time domain spectroscope device, for example.